Spark plug and method of manufacturing same

ABSTRACT

A spark plug and a method of manufacturing same is provided a spark plug for an internal combustion engine comprising, a center electrode  2 , a ground electrode  3  is formed a spark discharge gap G with the center electrode  2 , a housing welded to the end portion  31  of the ground electrode  3 . The surface of the housing  4  is formed a plated layer  5  whose melting point is lower than the melting point of the housing  4  and is lower than the melting point of the ground electrode  3 . There is substantially no the plated layer  5  on a junction boundary face between the housing  4  and the ground electrode  3 . The burr including constituents of the plated layer  5  lies next to the junction boundary face  11.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2005-305993 filed on Oct. 20, 2005, the disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a spark plug for use in an internal combustion engine of an automobile, cogeneration and so on, and to a method for manufacturing the same.

BACKGROUND OF THE INVENTION

In the combustion chamber of internal combustion engine for use in automobiles, cogeneration and so on, a spark plug ignites a mixture gas of fuel and air by firing spark discharge. The spark plug has a center electrode, a ground electrode for forming spark discharge gap with the center electrode, and a housing welded to the end of the ground electrode.

Since spark plug is exposed to an environment that has severe heat load, the ground electrode is made of a Ni-based alloy having excellent heat-resistance and acid-proof property such as Inconel 600™. On the other hand, the housing that holds the center electrode is generally made of a low-carbon steel material.

The surface of housing made of low-carbon steel material is plated for corrosion protection. Plating the surface of the housing is disclosed, for example, in U.S. Pat. No. 6,750,597 (Japanese Patent Laid-open Publication No. 2001-68250), which describes that after the housing is welded to the ground electrode, the side of the ground electrode is masked and then the surface of the housing is plated.

However, in the case of this plating process, since the ground electrode is not completely covered by the masking material, there is a concern that a plated layer is adhered in the range of 1-2 mm from the end portion of the ground electrode on the welded side end of thereof. In such case, the adhered plated layer is peeled by thermal stress during engine operation and the peeled plated layer may cause flying sparks or lifted flame and causes a fear of an ignition at the end. Furthermore, as this process includes a masking process and multiple steps, this process also tends to increase costs.

In case of using an electroless Ni plaiting process for forming the plated layer, when the phosphor as an impurely in the plated layer remains on the junction boundary face, it causes a concern that there will be cracks generated during the process of cooling the welding portion. There is the concern that the ground electrode will peel at the base point of cracks portion while using the spark plug and cause ignition failure.

To solve these the problems, in U.S. Pat. No. 6,819,033 (Japanese Patent Laid-open Publication No. 2003-59617), as shown in FIG. 17 and FIG. 18, after the plated layer 95 on the welding portion between ground electrode 93 and housing 94 is removed, the ground electrode 93 is welded on the removed plated layer portion 99.

Thus, ground electrode 93 is welded to the top surface 941 of the housing 94 that has plated layer 95, after removing a part of the plated layer.

The problem of this process is that there is an increase in cost because it has multiple steps, furthermore, there is a lack of connection reliability between housing 94 and ground electrode 93.

The latter point is described hereunder in detail.

The removed plated layer portion 99 is desirably formed on only the junction boundary face portion between the ground electrode 93 and the housing 94. In fact, as shown FIG. 18, the plated layer 95 must be removed including an area outside of the junction boundary face to completely remove the plated layer 95 on the junction boundary face. Therefore, as shown FIG. 11, if the removed plated layer portion 99 that remains out side of the ground electrode 93 is not completely protected by the welding burr 96, rust will on gather that portion. When welding burr 96 cannot completely cover the removed plated layer portion 99, or even if it can do, when adhesion strength is not enough, it may corrode on the housing 94 and, if worst comes to worst, the ground electrode may peel off.

On the other hand, if the amount of plated layer removed by grinding in advance is not enough, the junction strength is low and there is the concern that the ground electrode 93 can easily separate from the junction boundary face. Especially, in the case of the electrolytic plating Ni-layer as using the plated layer 95, as the melting point of the plated layer 95 is high, the plated layer 95 easily remains on the junction boundary face and described problem can easily occur.

For this reason, since the accuracy of the plated layer removing process must be improved, this increases in cost too.

SUMMARY OF THE INVENTION

In view of the above-described problems, it is an object of the present invention to provide a spark plug having an improved structure capable of having a weld that is superior in mechanical strength between the ground electrode and the housing and of capable of surely preventing flame-off and ignition failure.

This object can be achieved by providing a spark plug comprising: a center electrode; a ground electrode forming a spark discharge gap with said center electrode; a housing welded to an end portion of said ground electrode; and a plated layer formed on a surface of said housing, said plated layer having a melting point lower than a melting point of said housing and a melting point of said ground electrode; substantially no plated layer exists on either of a junction boundary face between said housing and said ground electrode, or on a surface of said ground electrode; and constituents of said plated layer exists in a welding burr formed next to said junction boundary face.

In another embodiment, the method is provided for manufacturing a spark plug including a center electrode; a ground electrode forming a spark gap with said center electrode; and a housing welded to an end portion of said ground electrode; the method including forming a plated layer on a surface of said housing, said plated layer having a melting point in lower than a melting point of the housing and lower than a melting point of the ground electrode, welding between said housing and said ground electrode by resistance welding while contacting an end portion of said ground electrode with a top surface of said housing via said plated layer; and extruding out said plated layer from a junction boundary face between said housing and said ground electrode during said welding.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings, in which:

FIG. 1 is a cross-section of view of a top portion of a spark plug according to a first example embodiment of the present invention;

FIG. 2 is a side view of the spark plug of FIG. 1;

FIGS. 3A, 3B, and 3C illustrate a method of manufacturing a spark plug according to the first example embodiment of the present invention;

FIG. 4 illustrates resistance welding between the housing and the ground electrode according to the first example embodiment of the present invention;

FIG. 5 is cross-sectioned view of a junction boundary face between a housing and a ground electrode of spark plug according to the first example embodiment of the present invention;

FIG. 6 is a cross-sectioned view of a welding burr according to the first example embodiment of the present invention;

FIGS. 7A, 7B, 7C, and 7D schematically illustrate a boundary between a welding burr and a housing according to a second example embodiment of the present invention;

FIGS. 8A, 8B, 8C, and 8D schematically illustrate a joint boundary between a ground electrode and a housing according to the second example embodiment of the present invention;

FIG. 9 is a cross-sectioned view of a top portion of a spark plug according to a third example embodiment of the present invention;

FIG. 10 illustrates of resistance welding between a housing and a ground electrode according to the third example embodiment of the present invention;

FIG. 11 schematically illustrates, in cross section, view of the amount of sinking a housing into a ground electrode according to the third example embodiment of the present invention;

FIGS. 12A, 12B, and 12C schematically illustrate, in cross section, the amount of welding burr at the time of weld between a housing and a ground electrode to fix according to the third example embodiment of the present invention;

FIG. 13 is a graph representing a relationship between the amount of sinking of a housing and a position of remaining phosphor according to the third example embodiment of the present invention;

FIG. 14 schematically illustrates a method of manufacturing a spark plug according to a fourth example embodiment of the present invention;

FIG. 15 schematically illustrates a method of manufacturing a spark plug according to a fifth example embodiment of the present invention;

FIGS. 16A, 16B, 16C, and 16D illustrate a method of manufacturing a spark plug according to a comparative example;

FIG. 17 illustrates a method of manufacturing a spark plug according to a prior art;

FIG. 18 illustrates removing a portion according to a prior art;

FIG. 19 illustrates covering a removed plating portion with a welding burr according to a prior art;

FIG. 20A is a cross-sectioned view of an example of a ground electrode, and FIG. 20B is a cross-sectioned view taken along the line XXB-XXB in FIG. 20A; and

FIG. 21A is a cross-sectioned view of another example of a ground electrode, and FIG. 21B is a cross-sectioned view taken along the line XXIB-XXIB in FIG. 21A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1

An example embodiment of the method of manufacturing of a spark plug for an internal combustion engine of the present invention will be described hereunder with reference to the accompanying drawings, such as from FIG. 1 to FIG. 6.

A spark plug 1 for an internal combustion engine according to an example embodiment of this invention defined a spark discharge gap G between a center electrode 2 and a ground electrode 3. An end portion 31 of the ground electrode 3 is welded to a housing 4.

A plated layer 5 whose melting point is lower than the melting point of the housing 4 and the melting point of the ground electrode 3 is formed on the surface of the housing 4.

The plated layer 5 is absent from a junction boundary face 11 between the housing 4 and the ground electrode 3 and on the surface of the ground electrode 3.

A welding burr 12 lying next to the junction boundary face 11 includes the constituents of the plated layer 5.

The plated layer 5 has the melting point of 1000° C. or less. More specifically, according to one example embodiment, the plated layer 5 consists the electroless nickel plating having a melting point of 890° C.

The housing 4 is made of an iron based metal and at least the outermost layer of the ground electrode 3 is made of heat-resistant nickel alloy. In one Example, the housing 4 is made of low-carbon steel and at least the outermost layer of the ground electrode 3 is made of Inconel 600™.

The spark plug 1 is mounted on an internal combustion engine, such as automobile engine or the like. The housing 4 is formed with a screw portion 44 to securing the spark plug 1 to the internal combustion engine, an insulator 13 is secured inside the housing 4, the center electrode 2 is secured inside the insulator 13, and the ground electrode 3 is welded to the top surface 41 of the housing 4. A spark discharge gap is formed between the top portion of the center electrode 2 and the ground electrode 3. Noble tips 22, 32 such as Pt, Ir and so on are formed on the top portion of the center electrode 2 and on the center electrode facing surface of ground electrode.

The method of manufacturing of the spark plug 1 for an internal combustion engine of this invention will be described hereunder with reference to drawings of FIGS. 3A-3C.

As shown in FIG. 3A, a plated layer 5, whose melting point is lower than the melting point of the housing 4 and the melting point of the ground electrode 3, is formed on the surface of the housing 4. In this Example, the plated layer 5 made of electroless nickel plating whose melting point is 890° C. is formed on the surface of a housing 4 made of low carbon steel S25C whose melting point is 1589° C.

After that, as shown in FIG. 3B, the end portion 31 of the ground electrode 3 directly touches the top surface 41 of the housing 4 via a plated layer 5 and is welded there to by resistance welding. At this time, the housing 4 and the ground electrode 3 are joined while extruding out the constituents of the plated layer 5 from the junction boundary face 11 between the housing 4 and the ground electrode 3. In this example, the ground electrode 3 is made of Inconel 600 whose melting point is 1425° C.

As shown in FIGS. 5, 6, the welding burr 12 that includes phosphor (P) as the constituents of the plated layer 5 is formed next to the junction boundary face 11.

In welding process, as shown in FIG. 4, the housing 4 is received on a lower electrode 61 and the ground electrode 3 is clamped by a pare of upper electrodes 62. The end portion 31 of the clamped ground electrode 3 is directly contacted with the top surface 41 of the housing 4 and is pressed against the top surface 41.

In this situation, the current and press conditions for the resistance welding are selected so that phosphor (P) can be extruded from the junction boundary face 11 alones with the other constituents of the plated layer. In this Example, the ground electrode 3 whose shape of cross section is 1.6×4.1 mm of substantially rectangle is welded to the following condition; current 2.9 kA, cycle 20, load applied between electrodes 45 kgf.

As shown in FIG. 3C, the ground electrode 3 is bended and the center electrode 2 held by the insulator 13 is disposed through the inside of the housing 4. The spark discharge gap G is formed between the center electrode 2 and the ground electrode 3.

The function and result of this Example will be described.

In the above method, the plated layer 5 is extruded from the junction boundary face 11 between the housing 4 and the ground electrode 3. Therefore, crack caused by the impurities (phosphor) in plated layer 5 can be prevented in solidification. Thus, the ground electrode 3 can be prevented from separate and junction strength can be ensured.

In addition, since the surface of the ground electrode 3 has substantially no plated layer, so that flying sparks and lifted flame resulting from peeling the plated layer by a thermal stress can be prevented, and a fear of a resulting ignition failure can be reduced.

Thus after the plated layer 5 is formed on the surface of the housing 4, and without the need to eliminate a portion of the plated layer, resistance welding is performed while directly touching the end portion 31 of the ground electrode 3 on the top surface 41 of the housing 4 via a plated layer 5. For this reason, masking the ground electrode 3 before forming the plated layer 5 is not required, nor is the process of eliminating the plated layer at the welding portion. For this reason, the manufacturing cost is reduced so that an inexpensive spark plug 1 can be provided.

As the melting point of the plated layer 5 is also lower than the melting point of the housing 4 and the melting point of the ground electrode 3, the plated layer 5 that is exited between the housing 4 and ground electrode 3 directly touching the top surface 41 of the housing is melted at the beginning of during in the resistance welding process. As a result, the plated layer 5 can be extruded out from the junction boundary face 11. Jointing the housing 4 and the ground electrode 3 while the extruding out from the junction boundary face 11 ensures the junction strength between the housing 4 and the ground electrode 3.

The extruded constituents of the plated layer 5 remains in the welding burr formed on the next to the junction boundary face 12. Even if the welding burr 12 includes the constituents of the plated layer 5, it dose not influence to the junction strength between the housing and the ground electrode.

As explained above, after the plated layer 5 is formed on the whole surface of the housing 4, the ground electrode 3 is welded to the housing 4. For this reasons the plating is easily prevented from adhering the plating on the ground electrode 3 without fail. Accordingly, lifted flame resulting from peeling the plated layer from the ground electrode 3 due to thermal stress can be prevented and then a spark plug that can reduce the likelihood of an ignition failure can be provided.

As the plated layer 5 of this example consists of electroless nickel-plating, the melting point of the plated layer 5 can be sufficiently to be lower than that of the housing 4 and that of the ground electrode 3. Accordingly, the plated layer 5 can be easily extruded out from the junction boundary face 11.

Namely, when the housing 4 is made of low carbon steel (melting point 1539° C.) and the ground electrode 3 is made of Inconel 600 (melting point 1425° C.), the plated layer 5 that is made of electroless plated nickel has melting point that differs by 500° C. or more as compared to the ground electrode 3 and the housing 4. For this reason, the plated layer 5 can be easily extruded out from the junction boundary face 11.

For example, when the core of the ground electrode 3 is made of copper (melting point 1083° C.) that has high coefficient of thermal conductivity material, the plated layer 5 (melting point 890° C.) has a melting point that differs by 180° C. or more as compared to the ground electrode 3 and the housing 4. For this reason, the plated layer 5 can be also easily extruded out from the junction boundary face 11.

As follows, examples of ground electrodes are described. As shown in FIGS. 20A, 20B, an example of a ground electrode 200 has a structure constructed of two-layers of different materials. For example, the ground electrode 200 includes a core 220 that is surrounded by an outermost layer portion 210. The core 220 may be made of copper. The outermost layer portion 210 may be made of heat-resistant nickel alloy. As shown in this example, the center portion of the ground electrode may be made of substantially single material such as copper.

Alternatively, as shown in FIGS. 21A, 21B, another example of a ground electrode 300 has a structure constructed of three-layers of different materials. For example, the ground electrode 300 includes a core 330, a middle layer portion 320, and an outermost layer portion 310. The core 330 is surrounded by the middle layer portion 320, which is further surrounded by the outermost layer portion 310. The core 330 may be made of pure nickel. The middle layer portion 320 may be made of copper. The outermost layer portion 310 may be made of heat-resistant nickel alloy. As shown in this example, the center portion of the ground electrode may be made of multiple materials.

The spark plug 1 can be used for the internal combustion engine of automobile, cogeneration and so on.

In a preferred embodiment of the above aspect, the melting point of the plated layer 5 is 1000° C. or less.

In this case where, the plated layer 5 can be easily extruded out from the junction boundary face 11 without fail.

There is copper (melting point is 1083° C.), such as having low melting point that is generally used among the housing 4 and the ground electrode 3 for the spark plug 1. When the melting point of the plated layer 5 is 1000° C. or less, even if the ground electrode 3 is made of copper, since the plated layer 5 is melted the beginning of welding process, the plated layer 5 is easily extruded out from the junction boundary face 11 and the plated layer 5 can be prevented from remaining in the junction boundary face 11. Especially, the plated layer 5 preferred to have a melting point that differs by 180° C. or more as compared to housing 4 and the ground electrode 3. Furthermore, more preferred to be the difference by 400° C. or more.

In a preferred embodiment of the above aspect, the plated layer 5 is made of electroless nickel plating.

Namely, when the housing 4 is made of low-carbon steel (melting point 1539° C.) and the ground electrode 3 is made of Inconel 600 alloy (melting point 1425° C.), the plated layer 5 made of electroless nickel plating (melting point 890° C.) has a melting point difference of 500° C. or more from the ground electrode 3 and the housing 4. If, for example, the ground electrode 3 is made of copper (melting point 1083° C.), the plated layer 5 has a melting point difference from the housing 4 and the ground electrode 3 of 180° C. or more. For the reason, in the welding process, the plated layer 5 is melted before the housing 4 and the ground electrode 3 without fail, and the plated layer 5 can be easily extruded out from the junction boundary face 11.

The plated layer 5 is made of electroless nickel plating.

In this case, since the melting point of the plated layer 5 can is sufficiently lower than the melting point of the housing 4 and the ground electrode 3, the plated layer 5 is easily extruded out from the junction boundary face 11.

The housing 4 is also made of low carbon steel such as a steel including iron, the ground electrode 3 is made of Inconel such as a heat resistant nickel alloy. Therefore, the housing 4 can have good formability and be low in price, and the ground electrode 3 can have a high resistance to heat and oxidization at the same time.

Furthermore, the steel including metal of the housing 4, low-carbon steel, stainless and so on. The ground electrode 3 can be made of nickel base alloy for example, Inconel 600™, Inconel 601 and so on. The ground electrode 3 can be made of high thermal conduction metal, such as copper (Cu) and so on that is covered over by a heat resistant nickel alloy.

In this way, this Example can provide the spark plug 1 that has high junction strength between the housing 4 and the ground electrode 3 without the process of masking the ground electrode 3 while forming the plated layer 5, and without the process of removing the plated later 5 at the only welding portion. Accordingly, the manufacturing cost of the spark plug c1 an be reduced and, a low price spark plug can be provided.

As aforementioned, after the plated layer 5 is formed on the surface of the whole of the housing 4, the ground electrode 3 can be welded to the housing 4, and adherence of the plated layer 5 on the ground electrode 3 can be easily prevented without fail.

As mentioned above, with this Example, an inexpensive method manufacturing of the spark plug 1 for an internal combustion engine, that has a high junction strength between the housing 4 and the ground electrode 3 and, decreased ignition failure, can be provided.

Example 2

FIG. 6 is cross-sectioned view of the welding burr 12. FIG. 7A is enlarged view of dot-line frame A, namely, FIG. 7A is view of side of the boundary between the housing 4 and the vicinity of the end portion of far from side of the ground electrode 3 at welding burr 12. FIGS. 7B, 7C, and 7D are views mapping the amount of phosphor (P), nickel (Ni) and iron (Fe). FIG. 7 is pattern view of patterning a frame format of SEM's photograph and images of mapping in the dot-line frame A of FIG. 6. In FIGS. 7B, 7C, and 7D are views indicating by hatching the relative density of each constituent (P, Ni, Fe) and the relative density represented by each hatching is shown in the explanatory note of FIG. 7E.

FIG. 8A is an enlarged view of dot-frame B in FIG. 6, namely, the junction boundary face 11 therefore. FIGS. 8B, 8C, and 8D are views mapping the amount of phosphor (P), nickel (Ni) and iron (Fe). FIG. 8 is also pattern view of patterning a frame format of SEM's photograph and of mapping image in the dot-line frame B of FIG. 6. The hatching portion of FIG. 8B, BC, and 8D are also indicate the relative density of the represented by each hatching is shown in the explanatory note of FIG. 8E.

With reference to FIG. 8B, phosphor (P) do not exist in the junction boundary face 11 between the ground electrode 3 and housing 4. On the other hand, with reference to FIG. 7B, thin concentration of phosphor (p) can be showed inside of the boundary between the housing 4 and the vicinity of the end portion far from side of the ground electrode 3, at welding burr 12. In FIG. 7B, two phosphor (P) layers are distributed in the under and upper sides, the under side layer is shown in phosphor (p) in the plated layer 5 on the surface of the housing 4, the thickness of the layer is 6±2 μm. Phosphor (p) that distributed in the upper side layer is phosphor (p) in the welding burr 12.

The result of a quantity analysis of the area of 5×5 μm˜10×10μ which is enlarged of multiple portion of the portion, that Phosphor (p) is distributed in the upper side layer, was detected 5˜15 wt % of phosphor (p).

On the other hand, the other portion of FIG. 7B was detected. In the result, the result of the quantitative analysis was detected 0.01 wt % or less of phosphor (p). The result of the quantitative analysis of phosphor (p) in FIG. 8B was 0.01 wt % or less.

As mentioned above, in this Example, it can be confirmed that the plated layer 5 in the junction boundary face 11 is melted and concentrated in the vicinity of the top of the welding burr 12 during formation of the welding burr 12 and then extruded out.

In this Example, the welding burr is formed next to the junction boundary face 11 between the housing 4 and the ground electrode 3 and the resistance welding is carried out so that welding burr 12 includes all the constituents of said plated layer 5.

In this cases where, the constituents of the plated layer 5 that exit from between the housing 4 and the ground electrode 3 are extruded out from the junction boundary face 11 with the welding burr 12. Even though constituents of the plated layer 5, exist in the welding burr 12, including impurities such as phosphor, it dose not influence the junction strength between the housing 4 and the ground electrode 3. For this reason an inexpensive spark plug that has a high junction strength between the housing 4 and the ground electrode 3 and reduced ignition failure is easily provided.

Example 3

In this Example, with reference to FIGS. 9-13, for example, this spark plug is comprised of a projection top portion 43 that is formed on the partial portion of the top side of the housing 4 and, the ground electrode 3 that is jointed with the projection portion 43 in the state of setting down at right angle against the axis direction of the housing 4.

A top portion 431 having a smaller cross section is formed on the top of the projection portion 43.

With reference to FIG. 11, in this Example, the housing 4 is also sunk into the ground electrode 3 by about 0.3 mm or more, and welded to the ground electrode 3. This amount of sinking H is defined as the amount from the standard position in a state of direct contact between the housing 4 and the ground electrode 3 before welding, to the position of the ground electrode 3 against the housing 4 after welding.

With reference to FIG. 10, in the welding process, the housing 4 is built in a lower electrode 61 and the ground electrode 5 is disposed on the projection top portion 43 of the housing 4 and pushed against the upper electrode 62 via the housing 4 and then, it is welded to the following condition of resistance welding; current 2.4 kA, cycle 20, load applied between electrodes 45 kgf.

In one example, the shape of the projection portion 431 of the projection top portion 43 is as follows; width w=11.0 mm, amount of projection u=0.2 mm. Thickness t of the ground electrode 5 is 1.6 mm, the distance s from the end surface of the ground electrode 5 to the projection potion 431 is 6.0 mm.

The other lengths are the same as Example 1. In FIG. 9-12, the plated layer 5 has been omitted. However, the plated layer is provided as in Example 1.

In this Example, since welding current is constricted to the top potion 431 of the projection portion 43 formed in a small cross section, the current density increases, and the plated layer 5 formed on the top surface 41 of the housing 4 is easily melted and can be extruded out.

Since it is heated at the certain point at first, the top portion 431 can reduce fluctuation of the junction strength between the housing 4 and the ground electrode 3. More particularly, since the position of the top portion 431 is formed on the most loaded portion, the welding strength of the portion can be ensured, and the reliability of the welding can be improved.

A projection portion can be formed on the end portion of the ground electrode 3 and, the projection portion of the ground electrode 3 contacts the top surface of the housing 4 while the ground electrode 3 is welded to the top surface of the housing 4 by resistance welding.

Since low current can easily heat welding portion, the plated layer 5 can be easily extruded out from junction boundary face 11.

By sinking the housing 4 into the ground electrode 3 by about 0.3 mm or more, since a sufficient amount of the plated layer 5 can be easily extruded out from junction boundary face 11, the junction strength between the housing 4 and the ground electrode 3 can be ensured.

Namely, since the top surface 41 of the housing 4 is sunk into the ground electrode 3 by about 0.3 mm or more, with reference to FIGS. 12A-12C, the plated layer 5 that exits on the top surface 41 of the housing 4 is extruded out with evolving welding burr 12 during the resistance welding process between the housing 4 and the ground electrode 3. With reference to FIG. 12C, the welding burr 12 is extruded around the joint boundary burr 11, the constituent (phosphor) of the plated layer 5 is distributed in the welding burr 12.

Also, the plated layer 5 can be smoothly extruded out because the ground electrode 3 is disposed on the top portion 431 and welded by resistance welding. Namely, with reference to FIGS. 12A, 12B, the gap 15 between the housing 4 and the ground electrode 3 that is formed on the side of the top portion 431 forms the route of elimination of the welding burr 12. And, the welding burr 12 is easily extruded out of the junction boundary face 11 via the gap 15 while the housing 4 sinks.

With reference to FIG. 13, in this Example, the relation between the amount of sinking of the housing H and the position of remaining Phosphor (p) as the constituent of the plated layer 5 is confirmed.

With reference to FIG. 13, when the amount of sinking H is less than 0.3 mm, there is Phosphor (p) in the junction boundary face 11. When the amount of sinking H is larger, the existing area is smaller. But when the amount of sinking H is 0.3 mm or more, there is no Phosphor (p) in the junction boundary face 11.

The amount of sinking is prescribed by the distance from the situation of touching between the housing 4 before welding to the position of the ground electrode 3 against the housing 4 after welding.

As mentioned above, when the amount of sinking is 0.3 mm or more, the constituent (Phosphor) of the plated layer 5 is prevented from remaining in the junction boundary face 11, and the ground electrode 3 can be ensured to have sufficient junction strength.

Namely, when the amount of sinking of the ground electrode 3 into the top surface of the housing 4 is less than 0.3 mm, it may be difficult that the plated layer 5 is extruded out from the junction boundary face 11.

Since the plated layer 5 can be easily extruded out from the junction boundary face 11 between the housing 4 and the ground electrode 3 and, the junction strength between the housing 4 and the ground electrode 3 can be ensured.

The other of the effect and the result are the same as Example 1.

Example 4

With reference to FIG. 14, in this Example, the projection 45 is formed on the top surface portion 41 of the housing 4 and the ground electrode 3 is welded to the housing 4 by resistance welding while directly contacting the end portion 31 of the ground electrode 3 on the projection 45.

The shape of projection portion 45 is a triangle shape.

The other shapes are the same as Example 1.

In this Example case, whereby the shape of the projection portion 45 is a triangle shape, electroless nickel-plating that is melting along the slope of the projection portion 45 is easily extruded out while the welding burr 12 evolves.

As mentioned above, since this Example provides a shape such that the plated layer is easily extruded out, when the area of the junction boundary face between the housing 4 and the ground electrode 3 is large, this Example is particular helpful.

Since lower current than the case of no projection portion can heat the welding portion, the welding area of the ground electrode 3 is prevented from expanding more than necessity. Accordingly, the projection portion 45 can contribute to reduce fluctuation in the junction strength.

The other effects and the result are the same as Example 1.

Even if the shape of the projection portion 45 is, for example a semicircle, a rectangle and so on, the projection portion 45 can get an improved result.

Example 5

With reference to FIG. 15, in this Example, the projection portion 311 is formed on the end portion 31 of the ground electrode 3. The projection portion 311 of the ground electrode 3 is directly contacted with the top surface 41 of the housing 4 and then the ground electrode 3 is welded to the housing by resistance welding.

The shape of this example of the projection portion 311 is triangle shape, too.

The composition, the effect and the result is otherwise the same as Example 4.

Comparative Example

With reference to FIGS. 16A-16D, in this Example, according to the method of manufacturing a spark plug of the comparative example, after the ground electrode 3 is welded to the housing 4, the plated layer 50 is formed on the surface of the housing 4 by electrolytic nickel plating and the spark plug of this comparative can be obtained.

Namely, with reference to FIG. 16A, the ground electrode 3 that is made of Inconel 600 is welded to the housing 4 that is made of low carbon steel by resistance welding.

Next, at first, with reference to FIG. 16B, the surface of the ground electrode 3 is masked by masking material 91.

Next, with reference to FIG. 16C, after the plated layer 50 is formed the surface of the housing 4 by electrolytic nickel plating, the masking material 91 is peeled away.

Next, with reference to FIG. 16D, the ground electrode 3 is bent and the center electrode 2 held by the insulator is inserted inside of the housing 4. Bending the ground electrode 3 forms the spark discharge G between the center electrode 2 and the ground electrode 3.

As mentioned above, the spark plug of this comparative example is manufactured.

In the method of manufacturing a spark plug of this comparative example, as mentioned above, since masking material 91 need to be required, the masking step involves provide multiple steps and leads to increase in cost.

Since the ground electrode 3 is difficult to completely cover with masking material 91, the plated layer may be adhered in the range of 1-2 mm from the end portion in the welded side end of the ground electrode 3 (the end portion 31). In such case, the adhered plated layer that is formed on the welded side end of the ground electrode 3 (the end portion 31) is peeled by thermal stress during the engine operation and the peeled plated layer may cause flying sparks or lifted flame and lead to a fear of an ignition.

According to, this invention, as discussed previously, since there is not masking process, the manufacturing steps are reduced and an inexpensive spark plug can be provided.

Accordingly, after the plated layer is formed on the surface of the housing, the ground electrode is welded to the housing. Therefore, the plated layer is adhered on the ground electrode. There is concern regarding flying sparks or lifted flame.

Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. 

1. A spark plug comprising: a center electrode; a ground electrode forming a spark discharge gap with said center electrode; a housing welded to an end portion of said ground electrode; a plated layer formed on a surface of said housing, said plated layer having a melting point lower than a melting point of said housing and lower than a melting point of said ground electrode; substantially no plated layer exists on either of a junction boundary face between said housing and said ground electrode, or on a surface of said ground electrode; and constituents of said plated layer exists in a welding burr formed next to said junction boundary face.
 2. A spark plug according to claim 1, wherein said melting point of the plated layer is 1000° C. or less.
 3. A spark plug according to claim 1, wherein said plated layer is made of electroless nickel plating.
 4. A spark plug according to claim 1, wherein said housing is made of metal including iron and the outermost layer of the ground electrode is at least made of heat-resistant nickel alloy.
 5. A spark plug according to claim 1, wherein said housing is sunk into said ground electrode by about 0.3 mm or more, and welded to the housing.
 6. A method of manufacturing a spark including a center electrode; a ground electrode forming a spark gap with said center electrode; and a housing welded to an end portion of said ground electrode; the method including: forming a plated layer on a surface of said housing, said plated layer having a melting point in lower than a melting point of the housing and lower than a melting point of the ground electrode; welding between said housing and said ground electrode by resistance welding while contacting an end portion of said ground electrode with a top surface of said housing via said plated layer; and extruding out constituents said plated layer from a junction boundary face between said housing and said ground electrode during said welding.
 7. A method for manufacturing a spark plug according to claim 6, wherein the welding burr is formed next to the junction boundary face between the housing and the ground electrode and the resistance welding is carried out so that welding burr includes all the constituents of said plated layer.
 8. A method for manufacturing a spark plug according to claim 6, wherein a projecting portion is formed on said top surface of said housing and, said ground electrode is welded to said housing by resistance welding with said projection portion contacts said end portion of said ground electrode.
 9. A method for manufacturing a spark plug according to claim 6, wherein a projection portion is formed on said end portion of said ground electrode and, said projection portion of said ground electrode contacts said top surface of said housing while said ground electrode is welded to said top surface of said housing by resistance welding.
 10. A method for manufacturing a spark plug according to claim 6, wherein said plated layer is made of electroless nickel plating.
 11. A method for manufacturing a spark plug according to claim 6, wherein said housing is sunk into said ground electrode by about 0.3 mm or more, and welded to said housing. 